Railroad hopper cars which carry bulk particulate materials or commodity, such as grain, food products, plastic pellets, powder, or other materials, have long been in use. Hopper cars generally have one or more compartments for storing and transporting these materials. Each compartment has at least one hatch in the roof of the car at the top of the compartment to facilitate loading of the materials and at least one outlet at the bottom of the compartment for facilitating unloading of the materials. A hatch cover is provided for each hatch to close the compartment after loading and thereby prevent foreign matter and moisture from entering the compartment and contaminating the stored materials during transit. After the hopper car reaches its destination, the hopper car is typically unloaded using a vacuum conveying line which is attached to the outlet at the bottom of each compartment. As the vacuum draws the material out of the compartment, air must be drawn into the compartment to maintain the efficiency of the vacuum and to prevent damage to the hopper car which could result from the negative pressure created in the compartment by the vacuum pressure. Heretofore, the hatch cover at the top of the hopper car was manually opened to facilitate entry of air into the compartment. To open the hatch, a person climbed on top of the car, unlocked the hatch cover, and manually opened the hatch cover. Besides the obvious safety problems associated with manually opening the covers, the covers were sometimes left open, which resulted in the entry of contaminants into the compartment and damage to the hatch covers.
Heretofore, it has been known to provide a vented hatch cover for railroad hopper cars to solve these problems as illustrated by U.S. Pat. Nos. 4,819,830 and 5,064,089. The vented hatch cover facilitates the entry of air into the compartment of the hopper car during the emptying of the particulate matter from the bottom of the compartment without the need for manually opening and closing the hatch cover. One problem associated with the entry of air into the compartment through the vented hatch cover is that foreign matter and moisture tend to be suspended in the air and carried into the compartment thereby contaminating the particulate matter during transit. To solve this problem, air filters have been used in vented hatch covers; however, the filter must allow sufficient air flow into the compartment to avoid negative pressure situations. Accordingly, there is a need for a vented hatch cover which eliminates the safety problems associated with manually opening the covers during unloading and which allows sufficient air flow into and out of the hopper car while restricting the entry of moisture and other contaminants into the car.